Meeting Abstract
Sloths exhibit under-branch suspension by one or more limbs for extended periods of time. Despite these abilities that require great strength, their skeletal muscle mass is low; however, their overall limb form has been extremely modified for suspensory behaviors. One modification may be the homogeneous expression of slow myosin heavy chain (MHC) isoforms in muscle fibers with high fatigue resistance. This hypothesis was tested using a combination of protein gel electrophoresis (SDS-PAGE) and histo/immunohistochemical analyses to determine fiber type properties in the limb muscles of two-toed (Choloepus hoffmanni, N=3) and three-toed (Bradypus variegatus, N=3) sloths. A primary expression of only slow MHC-1 and fast MHC-2A is observed in each species. Both fiber types are reactive for oxidative potential, and all muscles analyzed indicate a broader proximodistal distribution of slow, oxidative fiber types, except for the carpal/digital flexors. Slow MHC-1 fibers are larger in cross-sectional area (CSA) than fast MHC-2A fibers. In addition, we observed that sloth muscles appear to have appreciable extracellular matrix in between regions of contractile tissue. The findings do not match our hypothesis, and collectively, they suggest that sloths have evolved slower-contracting, fatigue resistant muscles for suspensory behaviors. The physiological properties observed (e.g., larger CSA for MHC-1 fibers) are further consistent with economical force production. However, while slow fiber type specialization provides a means to sustain muscle force, large forces that may be required for suspension are likely more dependent on muscle architecture and potentially, the passive versus active force properties of their muscles.
